feat: add environment configuration loader

src/config/env.js

@@ -1 +1,44 @@
 // Environment variable loading and validation.
+
+export function loadConfig() {
+  const key = process.env.OPENAI_API_KEY;
+
+  if (!key) {
+    throw new Error('Configuration error: OPENAI_API_KEY is missing.');
+  }
+
+  const parseNum = (name, raw, fallback) => {
+    if (raw === undefined || raw === '') return fallback;
+    const n = Number(raw);
+    if (isNaN(n) || !Number.isFinite(n) || n < 0) {
+      throw new Error(
+        `Configuration error: ${name} must be a positive number, got "${raw}".`
+      );
+    }
+    return n;
+  };
+
+  const parseBool = (name, raw, fallback) => {
+    if (raw === undefined || raw === '') return fallback;
+    if (raw === 'true') return true;
+    if (raw === 'false') return false;
+    throw new Error(
+      `Configuration error: ${name} must be true or false, got "${raw}".`
+    );
+  };
+
+  return {
+    openaiApiKey: key,
+    openaiModel: process.env.OPENAI_MODEL || 'gpt-5.1',
+    temperature: parseNum('OPENAI_TEMPERATURE', process.env.OPENAI_TEMPERATURE, 0.2),
+    maxOutputTokens: parseNum('OPENAI_MAX_OUTPUT_TOKENS', process.env.OPENAI_MAX_OUTPUT_TOKENS, 2000),
+    logLevel: process.env.CHATGPT_MCP_LOG_LEVEL || 'info',
+    enableFileContext: parseBool('CHATGPT_MCP_ENABLE_FILE_CONTEXT', process.env.CHATGPT_MCP_ENABLE_FILE_CONTEXT, false),
+    contextDir: process.env.CHATGPT_MCP_CONTEXT_DIR || './context',
+    maxInputChars: parseNum('CHATGPT_MCP_MAX_INPUT_CHARS', process.env.CHATGPT_MCP_MAX_INPUT_CHARS, 30000),
+    maxFileChars: parseNum('CHATGPT_MCP_MAX_FILE_CHARS', process.env.CHATGPT_MCP_MAX_FILE_CHARS, 12000),
+    maxFiles: parseNum('CHATGPT_MCP_MAX_FILES', process.env.CHATGPT_MCP_MAX_FILES, 5),
+    maxLogChars: parseNum('CHATGPT_MCP_MAX_LOG_CHARS', process.env.CHATGPT_MCP_MAX_LOG_CHARS, 10000),
+    redactSecrets: parseBool('CHATGPT_MCP_REDACT_SECRETS', process.env.CHATGPT_MCP_REDACT_SECRETS, true),
+  };
+}